The role of the thyroid hormones thyroxine and triidodothyronine in the regulation of protein metabolism in vertebrate liver will be studied in relation to late fetal development and to metabolic adaptation in the adult. The principal experimental approach involves determination of rates of processes involved in the pathways of protein synthesis and secretion by rapid kinetic measurements in vivo. A new method which takes into account the heterogensity of size in polypeptide chains undergoing synthesis will be used to determine elongation rate in protein synthesis in liver of early and late fetal liver compared to adult. Dose dependency in the effects of thyroid hormones on total amino acid incorporation and on elongation rate will be examined in adult liver to attempt to distinguish multiple effects of the hormone on steps in the protein synthetic pathway. The correlation between levels of liver elongation factor 1 and rates of elongation determined in vivo will be examined in fish undergoing metabolic compensation to temperature change. The studies will be carried out in the Long-Evans rat and in the toadfish.